Is there a way to adjust the background image depending on the prediction? So in bad weather a picture with clouds, or in fair weather a picture with a blue sky? Where is the prediction information in CumulusMX?

If you use the station forecast then you would have to perform parsing of the forecast text to determine the image to use.

The forecast tag from Cumulus is a numeric tag, you could create a set of images that incorporate this number and use that to determine which image is displayed. I am sure other people have already done this, so try a search of the forum (probably using Google is best).

Read this viewtopic.php?f=14&t=1657 for inspiration - you may decide to use a forecast icon rather than change the background image - there is a link to some icons produced by a member towards the end.